The present invention is directed to a method and apparatus for improving the operation of a shunt regulator and, more particularly, to a method and apparatus for improving the control characteristics of a fuse protected shunt regulator.
Fuses are very important in protecting circuitry from overload conditions. They are designed to blow open at predetermined current levels and are selected based upon safety specifications designated for a particular circuit. A disadvantage associated with fuses is the lack of precise control over the activation of the fuse. The activation of a fuse does not occur within a narrow range of currents. Thus, the maximum hold current of a fuse could be substantially lower than the current required to open the fuse in a desired time period. The activation of a fuse is related to the thermal capacity of the fuse material and packaging and is measured in units of Amp2 sec (I2t). FIG. 1 is a graph illustrating a typical fuse activation profile for a fuse from a first batch and a fuse from a second batch. It can be seen from this graph that the range of currents that can activate the fuse is not narrow.
It is thus desirable to provide a fuse protected circuit that offers improved controllability over the activation of the fuse. In addition, it is desirable to provide a fuse protected circuit that offers redundancy in case of point defects in the fuse control circuitry.
In addition, fuse protected circuits are controlled by control circuits typically composed of switching circuitry. The control circuit needs to monitor the output voltage of the switching circuitry to determine whether a fuse has blown or not. This can be particularly important in feedback circuits where it is not desirable to allow open fuse nodes to load the feedback signal. Thus, it is desirable to provide a feedback circuit for a fuse protected circuit that isolates open fuse nodes.
According to a first aspect of the invention there is provided a shunt regulator. The shunt regulator includes a plurality of fuse elements, a plurality of pass elements an amplifier and a feedback means. Each fuse element has a first node and a second node wherein the second node of each fuse element is operatively coupled together. Each pass element has an input and an output wherein the first node of each fuse is operatively coupled to the output of one of the plurality of pass elements so that each one of the plurality of pass elements has an independent fuse associated therewith. The amplifier has an input and an output, the output of the amplifier coupled to the inputs of the plurality of pass elements. The feed back means is coupled to the first node of each fuse element for supplying a feed back signal to the input of the amplifier wherein the feed back signal is composed of the output from at least one first node that has an intact fuse associated therewith.
According to a second aspect of the invention there is provided a shunt regulator. The shunt regulator includes a plurality of fuse elements, a plurality of pass elements, an amplifier and a feedback circuit. Each fuse element has a first node and a second node wherein the second node of each fuse element are operatively coupled together. Each pass element has an input and an output wherein the first node of each fuse of operatively coupled to the output of one of the plurality of pass elements so that each one of the plurality of pass elements has an independent fuse associated therewith. The amplifier has an input and an output, the output of the amplifier coupled to the inputs of the plurality of pass elements. The feed back circuit is operatively coupled between the first nodes of each fuse element and the input of the amplifier. The feed back circuit isolates any node that has a blown fuse associated therewith from the input of the amplifier.
According to a third aspect of the invention there is provided a method of protecting a shunt regulator circuit using a bank of fuses having a plurality of fuse elements arranged in parallel wherein each fuse element has a first end operatively coupled to a power source or load and a second end operatively coupled to one of a plurality of nodes. The method includes the steps of: (a) operating the circuit in a shunt regulator mode; and (b) switching the mode of operation of the circuit if a safety concern has been detected to a fuse activation mode, the fuse activation mode includes the steps of detecting a voltage at each of the plurality of nodes, isolating any node that has a blown fuse associated therewith from a feedback signal and sequentially activating each fuse in the bank of fuses that has an intact fuse associated therewith until the safety concern is eliminated.
According to a fourth aspect of the invention there is provided a shunt regulator circuit having a bank of fuses to protect the circuit wherein the bank of fuses has a plurality of fuse elements arranged in parallel wherein each fuse element has a first end operatively coupled to a power source or load and a second end operatively coupled to one of a plurality of nodes. The circuit includes means for operating the circuit in a shunt regulator mode; and means for switching the mode of operation of the circuit if a safety concern has been detected to a fuse activation mode, the fuse activation mode comprising the step of sequentially activating each fuse in the bank of fuses until the safety concern is eliminated.
A more complete appreciation of the present invention and its improvements can be obtained by reference to the accompanying drawings, which are briefly summarized below, to the following detailed description of the presently preferred embodiments of the invention, and to the appended claims.